Not satisfied with the results, Royal Pingdom moved on to research firms, and found that Experian Hitwise said Pinterest had become the third-most-popular social network behind Facebook and Twitter. It also extrapolated data from a report on Pinterest's growth through March, and applied that to the last few months to estimate that it now outpaces Tumblr usage in the U.S.

But before Pinterest runs a victory lap, it's important to point out that the data Royal Pingdom compiled should not be considered exact. All of the services, as well as the research firms, are estimating traffic. Alexa, for example, relies on data collected from those who have downloaded and installed its toolbar. And Experian's information from March did not include mobile traffic. Furthermore, some of the metrics measure only unique visitors, while others, like Alexa's, take a look at the combination of page views and unique visitors.

Back in April, social-media marketer Tamba tried putting a figure to Pinterest's growth, saying that it had 4 million daily unique visitors coming to its service. That figure had doubled since January. Pinterest does not provide its own user data on its site.

Tumblr, however, does. And according to that company, it serves more than 16.4 billion monthly page views worldwide. It currently has nearly 62 million blogs and 26 billion posts.

It's clear that Pinterest and Tumblr are wildly successful. But trying to determine which is more popular in the U.S. without having raw data from both companies is extremely difficult and involves a lot of guesswork.

CNET has contacted Pinterest and Tumblr for comment on the report. We will update this story when we have more information.